MicroRNAs are small, 19-25mer RNAs that are encoded in the genomes of upper eukaryotes. MicroRNAs appear to regulate global gene expression by modulating the translation of mRNAs transcribed from key genes. Although the research field is still young, microRNAs have been implicated in early development, brain development, cell differentiation, apoptosis, and tumor progression. Like mRNA analysis, defining miRNA expression profiles in samples should make it possible to identify miRNAs that are involved in a wide variety of bioogical processes. Towards this end, we propose to create methods for miRNA isolation, labeling, and microarray analysis that will make it possible to catalogue the miRNA profiles of any sample. The procedures will incorporate labeling and array procedures that will allow existing microarray instruments to be used for miRNA analysis.